Build in Firefox Rated 3 out of 5 stars

It seems to be originally included in Firefox ver. 49.0.2 (and some lower versions probably):
about:config
extensions.youtubeautoplaystopper@grizzlyape.comNavigated
But setting true/false seems to do nothing...

Another command built-in Firefox:
media.autoplay.enabled
false - always stop autoplay
true - video opened in new tab is buffered only in background and start play automatically only after you view the tab. Opened in the same tab start play automatically.

Last way is use addon YouTube ALL HTML5 which have option to always stop autoplay or stop only in background (work like media.autoplay.enabled but not play automatically, play button must be pressed).

Rated 1 out of 5 stars

No work

No work Rated 1 out of 5 stars

No work

Firefox 38.0.1, Windows 8.1 x64 Rated 1 out of 5 stars

Only preliminarily reviewed, dead website, autopause of Youtube video not worked.

Rated 1 out of 5 stars

trash

Rated 1 out of 5 stars

Doesn´t work and opens a deadlink tab

Rated 1 out of 5 stars

ABSOLUTELY BULLSHIT!
Doesn't stop UTube autoplay, but opens deadlinks! Don't install

Rated 1 out of 5 stars

Malware. Users note it opens another tab when U close it & have 2 disable java 2 get rid of it = this is illegal.

Rated 1 out of 5 stars

This add-on doesn't work under Firefox 31.0. It installs, but Youtube videos still play automatically.

When you un-install the add-on, it opens a dead link:
http://www.grizzlyape.com/addons/youtube-autoplay-stopper/uninstalled/

Also has a nasty when you remove it Rated 1 out of 5 stars

It opens a url, which has a nasty javascript something which opens another tab when you close it. Need to disable javascript to get rid of it!

Rated 3 out of 5 stars

Please update it, it's dead

Stop working, looks like it have been neglected - DON'T TRY ! Rated 1 out of 5 stars

Stop working, looks like it have been neglected - DON'T TRY !

This user has a previous review of this add-on.

не работает Rated 1 out of 5 stars

гн

Rated 1 out of 5 stars

Yep. It USED to work but "USED" is the operative word, here. What it USED to do does not do a thing to fix the problem, today, and has not worked for a few months, now.

This user has a previous review of this add-on.

SmartVideo For YouTube 0.973 Rated 1 out of 5 stars

Don't bother installing Youtube AutoPlay Stopper no longer works with the new updated Youtube layout for Firefox v18 but I have found another Firefox add-on that does same thing called 'SmartVideo' by Ashish and go to preference then tick the "Save Bandwidth" box and now all the Youtube videos will stop auto playing until you are ready to press play button. Enjoy!

This user has 2 previous reviews of this add-on.

Rated 5 out of 5 stars

this was a great addon but youtube has disabled it so you get to watch their adds which are set to play before videos

Rated 1 out of 5 stars

R.I.P. Grizzly Ape

This user has 4 previous reviews of this add-on.

Rated 1 out of 5 stars

Не работает!!!

Rated 1 out of 5 stars

Doesn't work!

stopped working Rated 2 out of 5 stars

stopped working about 2 or 3 days ago.